Objectives of this study are to obtain a total of 70 primary and derived variables on 168 normal women at 5 year intervals beginning in 1967. The measurements consist of comprehensive calcium kinetic, calcium metabolic, and physiological measurements as well as radiogrammetric measurement of bone mass. The purposes are: 1. To obtain comprehensive longitudinal data on calcium metabolism, bone metabolism, and related physiological functions in a group of untreated women from the perimenopausal period to the median onset time of osteoporotic symptoms; 2. Thereby to provide an exhaustive physiological profile both on women who develop symptomatic osteoprorosis, and on those who do not, allowing characterization of the differences between them and of the factors influencing the differences; 3. Thereby permitting better identification of the osteoporosis prone subset and providing both a rational basis for prophylactic treatment and the identification of subjects who should be its target; 4. And finally, to characterize cross-sectional interrelationships between major physiological variables during this key era of osteoporosis development. BIBLIOGRAPHIC REFERENCES: Saville, P.D. and Heaney, R.P., Osteoporosis, Practical Geriatrics, pp. 367-380, H.P. von Hahn, Ed., Karger, Basal, 1975. Heaney, R.P., Saville, P.D., and Recker, R.R., Calcium Absorption as a Function of Calcium Intake, J. Lab. Clin. Med., Vol. 85, No. 6, pp. 881-890, June, 1975.